1. Field of the Invention
The invention relates to a seat load measuring system for use in a vehicle occupant protection device for securing the safety of a vehicle occupant and for measuring a seat load a vehicle seat bears or the weight of the occupant of a vehicle seat.
2. Description of the Prior Art
Vehicles have become equipped with seat belt systems and air bag systems as vehicle occupant protection devices. In order to enhance the performance of such devices, attempts are being made to control the operation of each vehicle occupant protection device in response to the weight of the occupant of a vehicle seat equipped with the vehicle occupant protection device as well as a detection of the occurrence of a vehicle crash. For example, the quantity of air bag deploying gas, the deploying speed of air bags and/or the pretension of seat belts are controlled in response to the occupant""s weight.
The present invention relates to vehicle seat load or occupant""s weight measuring systems used in vehicle occupant protection devices as mentioned above. For example, three of such seat load measuring systems are disclosed in European Patent EP0990565 and U.S. Pat. Nos. 6,039,344 and 5,971,432.
Generally, a seat load measuring system provided for a vehicle seat comprises four weight sensor assemblies and a hub assembly for collecting the weight sensor assembly outputs and providing an output signal determined by the weight of the occupant of the vehicle seat. FIG. 1 is a diagram showing a conventional way of determining the seat load component WLD posed on each of the weight sensors from the output W of the weight sensor.
In FIG. 1, the horizontal axis indicates time and the vertical axis indicates the output W of the weight sensor. It is assumed in this figure that the used sensors are capable of measuring not only the pressure (i.e., the positive pressing force forward the sensor) but also the negative pressure or the tension. The least upper bound and the greatest lower bound of the weight sensor output are indicated by dotted lines Wmax and Wmin, respectively. A weight level W0 indicates the output level of a weight sensor assembly measured without applying any load thereto. After installing a seat assembly that includes the seat load measuring system in a vehicle under manufacturing, the output W0+W1 of each weight sensor assembly that includes a seat weight component W1 applied to the weight sensor assembly, is stored as a zero-point in the weight measuring hub assembly. Thereafter, the seat load (i.e., the load applied to a seat) is obtained by summing up seat load components given by the four weight sensor assemblies used for the seat. The seat load component for each weight sensor assembly is calculated as Wxe2x88x92(W0+W1) from the output W of the sensor assembly and the stored zero-point W0+W1 for the sensor assembly.
However, the zero point of one or more weight sensor assembly may deviate far from the stored value W0+W1 to a value W0+W2 due to any cause such as a minor crash or distortion of any rigid member of the seat that is caused by the lowering of durability. In such a case, the inability to adjust the zero-point in conventional seat load measuring system results in the seat load component being still determined by the expression WWxe2x88x92(W0+W1) using the old correction value W1. Further, if the deviation W2 is too large, which means that the positive dynamic range (PDR) is narrower than a necessary range, then the output of the weight sensor may be saturated for a large load. In this case, the seat assembly needs a disassembly and adjustment or the substitution of a new one.
Therefore, what is needed is a seat load measuring system that provides an accurate seat load measurement by adjusting its zero point even when the zero point has significantly deviated.
What is further needed is a seat load measuring system that alerts the user if the zero point deviation exceeds a predetermined value.
According to an aspect of the invention, a seat load measuring system which is for use in a vehicle occupant protection device for a vehicle seat and which has a zero point adjusting capability is provided. The seat load measuring system comprises a plurality of weight sensor assemblies each for providing a first output indicative of a weight component of an occupant of the vehicle seat which component is applied to the weight sensor assembly; a hub assembly for collecting the first outputs and providing a second output determined by the first outputs; and means for communicating a user""s desire for a zero point adjustment to each of the weight sensor assemblies. Each weight sensor assembly includes a zero point adjusting section, which receives an output from the communication means for executing a zero point adjusting operation.
The predetermined zero adjusting operation includes providing an alert if a zero point of the weight sensor assembly is not within a predetermined range.
The user""s desire for a zero point adjustment may be communicated to the zero point adjusting section by using an input device for permitting the user to enter a predetermined command; and portions distributed in the hub assembly and each weight sensor assembly for transferring the predetermined command to the means for executing a predetermined zero adjusting operation.
Alternatively, the user""s desire for a zero point adjustment may be communicated to the zero point adjusting section by using a momentary switch provided in the hub assembly; and portions distributed in the hub assembly and each weight sensor assembly for transferring the operation state of the momentary switch to the zero point adjusting section.